1. Field of the Invention
The present invention relates to an image sensing apparatus and method which form an image by using radiation including visible light, X-rays, and the like (which will be generically called light in the present invention) and, more particularly, to a -one- or two-dimensional image sensing apparatus such as a facsimile apparatus, a digital copying machine, a still camera, or a radiation image sensing apparatus, and an image sensing method.
2. Description of the Related Art
Various image sensing apparatuses using large sensors each having a -one- or two-dimensional array of image sensing elements, each using a monocrystalline Si sensor typified by a CCD sensor and a MOS sensor or a PIN sensor consisting of hydrogenated amorphous silicon (to be referred to as a-Si hereinafter) have been proposed.
As these image sensing apparatuses, in addition to image sensing apparatuses designed to convert a visible light image into an electrical signal, image sensing apparatuses designed to convert a radiation image into an electrical signal have also been developed with development in nuclear technology and advances in medical radiographic devices and nondestructive testing.
Many of these image sensing apparatuses, however, have S/N ratios of the order of -2- and 3-digit figures, and have not been required to have higher S/N ratios for the following reasons. There have been no A/D converters suited to high-precision digitization of outputs with high S/N ratios. In addition, since the data amount after conversion becomes large, limitations are imposed on memories and communications, resulting in poor operability. This has reduced the necessity of image sensing apparatuses with high S/N ratios.
With the recent remarkable developments in large-capacity memories and high-speed communications, great demands have arisen for image sensing apparatuses with high S/N ratios of the order of -4- and 5-digit figures.
In general, however, a decrease in S/N ratio is inevitably caused by variations in the manufacturing process and variations in fixed patterns and sensitivity. To prevent this, the fixed pattern and sensitivity variations are stored in a memory, and correction is performed in actual operation by using the photographic output and the information in the memory.
In this technique, however, since the data used for correction is obtained before photographing operation, the following problems are posed.
In general, in using an image sensing apparatus, the user selects operation conditions for the apparatus in accordance with the purpose of photography or the like. In addition, the components of the image sensing apparatus vary in characteristics with changes in temperature. Furthermore, various types of automatic control such as automatic exposure and automatic gain adjustment are activated during photographing operation to easily obtain optimal images without mistakes. That is, the conditions actually set for the photographing operation differ from the conditions set to generate the data used for correction. Since fixed pattern and sensitivity variations that cause a drop in S/N ratio slightly vary depending on these conditions, errors contained in the photographic output cannot be corrected perfectly. As described above, the differences between the conditions set to obtain the data used for correction and the conditions set for the actual photographing operation may pose a problem in obtaining image information with a high S/N ratio.